The present invention relates to welding within the confines of a nuclear reactor pressure vessel (RPV) bottom head and more particularly to a robotic welder specifically designed for such welding purposes.
Cracking in the heat affected zone of stainless steel and Ni--Cr--Fe welds within a boiling water reactor (BWR) environment is a well-known problem. Repair of such cracks is performed by removing the cracked material and rewelding the components using improved materials and processes. When the cracked material is located in an inaccessible area, such as the RPV inside bottom head, special methods must be employed to affect the repair.
The stub tube is a weldment located on the inside surface of the RPV bottom head between adjacent control rod drive housings. The drive housings severely restrict access to the repair area. The core plate and top guide further restrict access. Because the BWR bottom head is hemispherical in shape, the stub tube centers will vary from near horizontal to a maximum in core housing penetration angle of about 46.degree., which geometry additionally complicates the repair process.